There is a case in which a thick film needs to be formed on a base material to manufacture a structure. For example, such a structure can be a combustion chamber of a rocket engine for aerospace. When the combustion chamber of the rocket engine is manufactured, a copper film having a film thickness equal to or more than 10 mm has to be formed on a copper base material.
As a method of forming such a thick metal film, “an electroplating method” is exemplified. However, a film growth rate by the electroforming method is very small, thereby taking several months to achieve a target film thickness of about 10 mm, for example.
To solve such a problem, the applicant of the present application proposed a technique of forming a metal thick film by using “a cold spray method”, in JP 2012-057203. The cold spray method is a method in which a high speed flow of gas is formed to have a temperature lower than a melting point or softening temperature of material powder, particles of the material powder are injected into the gas flow and accelerated, and the material powder particles are made to collide with a base material in a solid phase state. The film forming rate in the cold spray method is much faster than that of the electroforming method. Therefore, a period of time taken to manufacture the structure can be substantially reduced by using the cold spray method.
The inventors of the present invention found through an experiment that the following problems occurred when a thick film was formed by the cold spray method. The problems will be described with reference to FIG. 1 and FIG. 2.
As shown in FIG. 1, a film 30 is formed by the cold spray method on a film grown surface 10A which is the upper surface of a base material 10. The material of the film 30 is Inconel 718 of a Ni-based material. At this time, the area of the grown film 30 at a position became smaller as the position moved away from a boundary with the base material 10. In other words, the side surfaces 30S of the grown film 30 were inclinedly formed into a direction of the center from the side ends 10E of the film grown surface 10A. That is, it was found that the side surfaces 30S of the grown film 30 were formed in “a slope”.
FIG. 2 shows a case where the material of the grown film 30 is Cu. In this case, the area of the grown film 30 increased once and then decreased as it moved away from the boundary with the base material 10. It was found that the side surfaces 30S of the grown film 30 were still formed in “a slope” even in this case.
The phenomenon described above is not tangible in the case where a thin oxide film and so on is formed by the cold spray method and the problem is peculiar to the case where a thick film is formed by the cold spray method.